Natural Features and Ecosystems

Mangroves are critical habitats for reef and marine fishes, and for many species of invertebrates.

NPS photo.

Mangroves

Mangroves are tropical to subtropical intertidal forest ecosystems, subject to saltwater tidal immersion. While they can adapt to brackish conditions, and can be found in bays that serve as freshwater watersheds, they are not solely components of freshwater ecosystems. They are often found at the mouths of streams and rivers. Mangroves are “landbuilder communities,” they extend shorelines via systems of prop roots, trunks, pneumatophores, and saplings that trap and stabilize erosional terrestrial sediments. It has been reported that in some fringing mangrove systems, prop roots can result in anywhere from 25 to 200 meters of coastal accretion a year. In areas with high turbidity, fringe mangroves may act to slow sediment runoff and filter suspended solids. The accumulation of sediments furthers the process of deposition and shoreline extension, and elevates the land surface. Peats may also develop in low-energy environments. Because of their high organic content and calm waters, mangrove forests serve as nurseries for many species of both invertebrates and reef and marine fishes. They also provide subsistence for many species of crustaceans, wading birds, and small mammals. Mangrove forests are, therefore, extremely productive ecosystems with very high biodiversity.

The Salt River, Sugar, and Triton Bays contain mangrove forests that, prior to Hurricane Hugo in 1989, were among the most extensive in the U.S. Virgin Islands. Red (Rhizophora mangle), black (Avicennia germinans), and white (Laguncularia racemosa) mangroves are all found here, especially along the southern shores of Sugar Bay. Red mangroves fringe the immediate shorelines, and can advance in waters up to one meter deep. These mangroves can tolerate a variety of salinities, from fresh water to 44 parts per thousand (ppt). Because they cannot survive in highly saline environments, droughts can be deadly. Black and white mangroves form bands along shorelines. White mangroves become predominant in higher elevations with moist, silty soils. Black mangroves tend to replace white mangroves as soils become more saline, and are typically found on higher and drier soils than white mangroves.

 

A variety of corals grow on the walls of the submarine canyon at Salt River Bay.

NPS photo (S. Perhsern)

Coral Reefs and Submarine Canyons

Salt River Bay is protected from the sea by a barrier reef, composed primarily of finger corals and elkhorn corals; however, much of this reef is dead and overgrown with encrusting corals, sponges and algae. The offshore environment is dominated by a submarine canyon, composed of a shallow pavement shelf (30 to 50 feet; 9 to 15 m) that gradually drops into a deep forereef, ranging in depth from 50 to70 feet (15 to 21 m). The canyon then plummets to a depth of over 12,000 feet (roughly 3,650 m). The shelf consists of scattered elkhorn, brain, fire, boulder, and staghorn corals, while the walls are dominated by lettuce leaf corals (Agaricia spp.), sponges, and other soft corals; the canyon floor consists of seagrasses and seaweed beds.

Coral barrier reefs are extremely diverse ecosystems, with complex relations with other contiguous ecosystems (like seagrass beds). St Croix has several extensive barrier reef systems, complete with well-defined lagoons, not found on either St. Thomas or St. John. Over forty species of scleractinian corals have been recorded on the coral reefs of the Virgin Islands. These species include elkhorn coral (Acropora palmata), the primary reef-building coral of the entire Caribbean, star coral (Montastrea annularis), staghorn coral (Acropora cervicornis), and finger corals (Porites sp.)